Benzoic Acid by Polarized Light Microscopy


Benzoic acid is a colorless crystalline solid, or a simple aromatic carboxylic acid. The name initially comes from gum benzoin which was for a long time thought to be it’s only known source. Benzoic acid actually occured naturally in many plants where its function is typically as an intermediate for the biosynthesis of many secondary metabolites.

Salts of Benzoic acid are typically used as food preservatives, and the Benzoic acid itself is also an important precursor for the synthesis of many other substances including;

  • Benzoate plasticisers
  • Sodium benzoate
  • Benzyl chloride


How it was done:

Since Benzoic Acid has a relatively low melting point (<200 Deg C), it was prepared on a glass slide using the melt method. A small amount of analytical grade material was placed onto a concave slide, and gently heated over a bunsen burner within a fume cupboard; being careful not to break the slide or inhale the vapors. This was then left to crystallize on the slide and a cover slip placed on top.

The image was then captured using my normal polarized microscopy setup, which included;

Because the depth of field is relatively small, some images did require focus stacking which is where Helicon Remote & Focus came into the picture (pardon the pun).


Images: (Click to enlarge)